X-ray computed tomography systems (X-ray CT) designed for metrological use are frequently used in the manufacturing industry. This is because X-ray CT is able to measure not only outer geometry but also inner geometry nondestructively and relatively quickly. However, X-ray CT in the state of the art is not always able to demonstrate its measurement performance and traceability to SI. One of problems is that it is hard to evaluate error sources unique to X-ray CT, such as scattering and beam hardening, called as “material influence.” The hypothesis to the mechanism of Bi-directional length measurement error from the material influence is proposed. The hypothesis is that Bi-directional length measurement error is mainly caused by the form measurement error of a small feature on the gauge. The form measurement error of a small feature on the gauge is dominantly influenced by beam hardening. The hypothesis is validated through actual experiments and simulations. The results of the experiments and corresponding simulations lead us to the conclusion that the magnitude of the form measurement error of a small feature on a rotational asymmetric gauge is clearly correlated with a location of the small feature on the gauge.

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